The present invention relates to energy beam welding including electron beam and laser beam welding, for producing a weld seam in a work piece, wherein the energy beam and the work piece are moved relative to each other in the direction of the weld seam and the depth of a fused or welding zone (welding depth) in the work piece, measured in the direction of propagation of said energy beam, is varied along at least a portion of the weld seam by changing a welding parameter, generally the beam power.
When producing a weld seam by means of an energy beam, it is often necessary or advisable to start the welding operation with a relatively low beam power and to increase the beam power gradually to a final or nominal value while proceeding with the beam along the joint to be welded. The final or nominal value is then at least essentially maintained during the welding of the main portion of the weld seam or at most adjusted to cope with a varying work piece thickness. The "slope-in" region, during which the beam power is gradually increased and a fused or molten zone of gradually increasing depth is produced, has to be "overwelded" later to produce also in the slope-in region a welded zone which has throughout a final desired depth. Following to such overwelding, the beam power is then gradually reduced to produce a "slope-out" region, within which the remelted zone of the weld seam becomes gradually more shallow, until it eventually vanishes. Energy beam welding processes of this type are used mainly for producing a weld seam having the shape of a closed curve or loop, a typical example being the butt welding of two tube sections abutting with their front faces. The gradually increasing the beam power avoids inter alia that molten work piece material is thrown out of the welding zone at the beginning of the seam welding and is missing later. This is a severe problem, especially when work pieces in form of thick-walled hollow cylinders are to be joined by an energy beam, as an electron beam. Similar problems are generally also encountered when a weld seam is commenced in a work piece of relatively great thickness, and in repair welding operations, whereby only the slope-in region or the slope-out region may be required and the welding zone may extend to a portion of the work piece thickness only, and an overwelding may not be necessary.
It is general practice to periodically oscillate the energy beam at least in a direction transverse to the length dimension of the joint or weld seam when workpiece parts of greater thicknesses are welded, such oscillation being sometimes called "wagging". At the present state of the art, the parameters of such beam oscillation, e.g. the oscillation or deflection amplitude in a direction transverse and/or lengthwise to the longitudinal direction of the joint or seam, and the frequency of oscillation, were preset before the welding process is commenced and maintained essentially constant during the welding of the seam including the slope-in and slope-out regions. Other beam parameters, as the position of the beam focus in respect to the work piece, were also maintained constant throughout the welding.
In spite of a careful control of the variation of the beam power in the slope-in and slope-out regions, defects, as small voids and such a tendency of some materials to crack, could not be always prevented. Further, the weld seam did not have the desired uniformity of cross section and form of weld seam surface.
It is an object of the invention to provide a method of welding by means of an energy beam, as an electron beam, which is adapted to form loop-shaped welding seams, which inherently comprise a slope-in region which is overwelded in the course of the welding operation, and an slope-out region which remains unaltered; and elongated weld zones comprising individual slope-in or slope-out regions which may remain unaltered, where the quality standard of an uniform welding seam having its full depth can be essentially secured.